1. Field of the Invention
The present invention relates to a metal catalyst carrier for carrying a catalyst for purifying exhaust gases from automobiles or the like.
2. Description of the Related Art
Attention is drawn to a metal catalyst carrier for carrying a catalyst for purifying an automobile exhaust gas, that is produced by laminating and spirally winding together a flat foil of a heat-resisting stainless steel and a corrugated foil formed therefrom, joining the flat and corrugated foils at suitable contact portions thereof to form a honeycomb, inserting the honeycomb in a heat-resisting steel case, and joining the former to the latter.
It is important for a metal catalyst carrier to be resistive to the heat and oxidation because of hot exhaust gas and to the thermal stress and thermal fatigue because of repeated heating and cooling and temperature fluctuation over the honeycomb.
A solution to the problem of thermal stress and thermal fatigue because of repeated heating and cooling was disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 62-273050 and 62-273051, in both of which only the edges of the foils are joined to the case along the axial direction and the flat and corrugated foils of the honeycomb are not joined together.
Another solution was disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-83044, in which the flat foil has a wavy deformation with a large interval and the corrugated foil has additional corrugations with a small interval so that cells formed at the contact points between these flat and corrugated foils are allowed to deform to a greater extent, thereby mitigating the thermal stress.
The first solution, however, has a problem in that the flat and corrugated foils of the honeycomb are occasionally mutually displaced by a hot, rapid flow of exhaust gas, because these foils are joined to the case only in the edge portions.
The second solution has also problems in that it is difficult to impart a wavy deformation to a flat foil at a large interval, that a wavy-deformed foil is difficult to coil, and that the contact portions between the wavy-deformed and corrugated foils are difficult to stably join, causing an incomplete joining of each cell and failing to provide a stable structure of a honeycomb.
Japanese Unexamined Patent Publication (Kokai) No. 62-160728 disclosed a mechanical clamp of a honeycomb. This proposal, however, has a problem that the clamped honeycomb is separate from, or not joined to, a case and vibrates in the case to cause the carried catalyst to fall away, thereby reducing the purification ability.
It was also proposed to decrease thermal stress by improving a method of soldering flat and corrugated foils of a honeycomb. The improved method solders the flat and corrugated foils in selected portions of the honeycomb, i.e., the soldered portions have a vertical section either in a gate form when the foils are soldered in the upper end and in the several outermost layers of a honeycomb (herein referred to as "gate structure"), or in a symmetrical form when the foils are soldered in both ends of a honeycomb ("symmetrical structure").
Japanese Unexamined Patent Publication (Kokai) No. 62-45345 disclosed a honeycomb in which soldering is effected in a portion of a front face of the honeycomb in a regular or irregular pattern. This has an advantage that the solder material, which is generally expensive, can be saved by decreasing joints.
These proposals, particularly when applied to a recently proposed, short honeycomb, have also problems in that the "gate structure" causes a depression-type deformation to occur due to lack of stiffness in the middle portion of the honeycomb and that the "symmetrical structure" involves a poor stiffness of the honeycomb as a whole, and also, makes it difficult to provide a flexible structure composed of the honeycomb and the case with an optimum size relationship therebetween. The last proposal by Japanese Unexamined Patent Publication (Kokai) No. 62-45345 raises similar problems including a lack of the stiffness in the axial direction.
Japanese Unexamined Utility Model Publication (Kokai) No. 62-194436 disclosed that a honeycomb, in which flat and corrugated foils are joined together in one or a plurality of cross sections of the honeycomb, is joined to a case only in one of these cross sections to mitigate thermal stress in the axial direction. This structure, however, not only is difficult to apply to a short honeycomb but also has a problem in the durability under a radial thermal stress when considering a rigid structure formed at the joints between the honeycomb and the case.
As reviewed above, there are many proposals to solve the problems of thermal stress and thermal fatigue due to thermal cycle, but none of these conventional proposals is satisfactory because they unavoidably degrade the purifying ability of catalyst or raise a structural problem. The issues of thermal stress and thermal fatigue are particularly important in a short honeycomb, in which the temperature difference due to exhaust gases is greater in the radial direction than in the axial direction.
Special joined structures are difficult to apply to a short honeycomb. It is desirable that a short honeycomb has a rigid structure as a whole and the honeycomb and the case are joined by a flexible structure.